Brain Evolution II: Our Runaway Brain?

Runaway Brain? Skulls, Brains & Neotony

• Neoteny: retention of juvenile or infantile physical and behavioral traits into adulthood caused by a “slowing down” of physiological development

  • as humans evolved from common ancestor of chimps/primate, some genes related to skull devlopment similar to baby chimps may have allows for brain development

    • → allow for wiggle room for brain evolution

Hominin Brains: Rapid Evolution in Genus Homo

• from anthropoids to Australopithecus africanus => not much brain size

• from Australopithecus africanus to Homo habilis => large growth in brain size

  • something happened → maybe to brain or skull size

    • was very fast

• from Homo habilis to Homo sapiens => continuing to grow

• during 4 mya to 2 mya

  • lots of climate change happened → promotes selection pressures/challanges

    • could have contributed to large growth in brain size

Hominin Brains: Frontal Lobe Expansion

• Australopithicines: big expansion of the frontal lobes (set the stage for →)

  • could have been key to the evolution of complex cognition, problem-solving, language

    • expansion of frontal lobe was pushing us to be able to do this

  • which came first in hominins, language or tool-making?

    • language → doubtful; no evidence that they were smarter than chimps

    • tool-making → that may have came first bc there would be evidence left behind

• ex: A. Afarensis

  • more shallow/flat fl region

  • slows forehead, big brow ridge, heavy jaw

  • not large-vulting of skull

  • “Lucy’s” species

• ex: A. Africanus

  • more vulting of skull

Endocasts of Homo erectus & Homo sapiens

• homo sapiens is much larger than homo erectus

• Homo sapiens compared to homo erectus

  • PL has most prominent expansion

    • more tool use

    • spatial awareness → aided in more sophisticated tool use/making

    • somatosensory

  • FL were pretty similar

Hominin Brains: Parietal Lobe Expansion in Homo

• Homo erectus : characteristic "football-shaped" hominid skull/brain as frontal lobe expands (compared to other lobes)

  • not so much in PL

  • not as much vulting

• Homo Neanderthal & Homo sapiens: skulls/brains are more rounded & “vaulted”

  • show big expansion of the parietal lobes

  • ex Neanderthal skull: face more flat; more vulting

  • ex human skull: flat face; big/huge vulting

• Parietal lobe expansion correlates with advanced tool-making seen in cultures about 90,000 years ago

Contributions to Brain Evolution

• According to graph, greatest climate fluctuation correlates to greatest brain size increase

• last glaciar period

  • challenged ancesters to survive or die

  • favors larger brain

    • higher chance of survival

EQs for Australopithecines & Hominins

• EQ = encephalization quotent

  • calculates how big the brain compared to body

  • brain weight : body weight

• brain size correlates to body size

• brain body

• ex homo sapiens: large for primate of our size

But is Our Brain Extraordinary

• Human brain does have higher number of neurons compared to other primates & great apes; but likely in line with other Homo species

The Human Brain has lots of “support”

• ex mouse: has more neurons than glial cells

• ex human: more glial than neurons

  • way more helper cells

  • lots of neurons but w/ a lot of glial cells

Runaway Brain? Developmental Genes

• Microcephaly (“small brain”)

  • brain is sig. underdeveloped

  • sig. small

  • causes cognitive deficits

• Lissencephaly (“smooth brain”)

  • lack of wrinkles

  • cortex doesn’t fold

    • causes cognitive deficits

Cerebral Cortex Size, Density & NOTCH

• NOTCH gene expands cerebral cortex by...

  • keeping certain radial glial “stem-cells” active

    • stem cells prominant/active in early development

    • form scaffolding for high density neurons

      • make vines to help neurons move toward cortex

    • continue to divide

    • w/o gene → will settle/become not active anymore & become neurons

  • instead of converting to mature neurons, they continue to multiply

  • acts as scaffolding for more neuron development and migration

• keep neurons more active during brain development

  • allows for more dense cortex

• found in other mammals

  • but different in humans

Human Brain Characterized By Dense Connections

• synaptic pruning → fine-tuning of brain connections

• Human compared to chimp

  • way more dendrites, synaptics, myelination

  • way more dense connections

  • slower to develop

    • leads to more development & connections

NOTCH in Brain Size & Human Evolution

• Human-specific version NOTCH2NL can cause birth defect syndromes today from either duplications (macro-cephaly - “big brain”) or deletions (micro-cephaly - “small brain”) of NOTCH2NL

• Homo sapiens specific NOTCH2NL emerged about 3 mya by gene duplication, likely originating in LCA with Neanderthals, Denisovans

• NOTCH

  • associated w/ mutations that make the brain larger or smaller

  • fundamental in brain development in density & connections

Lissencephaly & ARHGAP

• ARHGAP → associated w/ gyrification & neuron proliferation

• ex: added human ARHGAP to fetal marmoset

  • w/ human version → larger in size, more bumps/wrinkles (gyriliferation)

Delayer Maturation in Human Brain

• starts out pre-mature → lots of development after brain

• if humans have big brains → happens post-birth so they can fit through birth cannel

• bc we have a brain that can learn → sponge for info (plasticity)

• ex:

  • mouse → brain development complete early in life

  • macaque → more extended brain development completion

  • chimpanzee → even more extended brain development completion

  • ^^^ for these examples brain is mostly finished pre-birth

  • human → much larger brain development into lifespan

    • before birth, right after birth, & post birth

Delayed Maturation & Brain Complexity

• ex: chimp

  • big rapid growth happens up to growth

  • @ 2 pretty much done

• ex: human

  • brain grows rapidly until age 2 from pre-birth

  • brain growth continues & slows until almost 6

  • @ 10 pretty much done

  • bulk is majority done in adelosence